1. Field of the Invention
The present invention relates to water-containing compositions, and in particular to aqueous polymer dispersions based on polychloroprene, to processes for their production and to their use, in particular for the production of coatings, and in particular for the production of adhesive coatings, and to a process for bonding substrates using the compositions.
2. Description of the Prior Art
Contact adhesives based on polychloroprene (CR) are predominantly solvent-containing adhesives which are applied to the substrates to be bonded and dried. By then joining the substrates under pressure a bonded structure is obtained having a high initial strength immediately after the joining process. The subsequent crosslinking produces the final curing.
For ecological reasons there is an increasing need for suitable aqueous CR adhesive dispersions which can be processed to form corresponding aqueous CR adhesive formulations. These are for example used in the “spray-mixing” process, in which the aqueous adhesive formulation and a coagulating agent are separately conveyed through a spray gun and finally mixed in the spray jet, after which the CR adhesive coagulates on the substrate. An overview of this method is provided for example in “Handbook of Adhesives”, Irving Skeist, Chapman Hall, New York, 3rd Ed. 1990, Part 15, page 301, R. Musch et al, Adhesives Age, January 2001, page 17, “Spray-Mixing Klebstoffe auf Basis Dispercoll®C für die Schaumstoff-Klebung” (Spray-mixing adhesives based on Dispercoll®C for the bonding of foams), Technical Brochure from Bayer AG, No. KA-KR-0001d/01/05.96.
It is however frequently necessary to add additives, such as for example stabilizers, to the aqueous CR dispersions or formulations, in order to ensure that they have the necessary stability in storage and reliability for use or that the adhesive layers are also protected from ageing or discolouration.
For the latter purpose zinc oxide is advantageously added to aqueous formulations, since, where formulations based on polychloroprene dispersions are used, zinc oxide counteracts rapid ageing of the bonded seam and discolouration of the bonded substrates by the cleavage of HCl from the CR polymer.
The zinc oxide types so far known do however tend to form sediments and the known zinc oxide dispersions are prone to phase separation (so-called “phasing”). This sedimentation or phase separation is not acceptable, particularly where such adhesive formulations are used in the “spray-mixing” process, since blockages are produced in the nozzle as a result. The cleansing of the nozzle is highly time-consuming and costly and economically unsatisfactory.
In order to solve the problem of the sedimentation of zinc oxide, various processes have been described for producing stable, non-segmenting zinc oxide dispersions.
DE-A 19 703 582 describes the production of a zinc oxide paste by processing a commercially available zinc oxide with a wetting agent to form an aqueous zinc oxide paste. This paste has a number of disadvantages for use in aqueous adhesive dispersions. On the one hand the paste is highly viscous and thixotropic and therefore difficult to process. Also, after the paste has been incorporated into the aqueous adhesive dispersion, its resistance to sedimentation is reduced, since the zinc oxide particles in commercially available zinc oxides are present in a highly agglomerated form and the agglomerates are not completely dispersed by the described process. Also, agglomeration means that the reactive surface area is only partially available so that relatively high starting quantities of zinc oxide have to be used to obtain the required effect of the HCl absorption.
WO95/24359 describes a colloidal zinc oxide and a process for producing colloidal solutions by grinding conventional, highly agglomerated zinc oxides in the presence of polyacrylic acid, the resulting colloidal solution containing agglomerates of <100 nm. This process has the disadvantage that a very intense grinding process is necessary for overcoming the powerful interactions of the agglomerated particles in order to obtain stable dispersions having agglomerate sizes of <100 nm. The process thus requires a very high degree of technical effort and a very high amount of energy, allows only very small throughput quantities and is therefore unsuitable for the economic production of industrial product quantities. Also, due to agglomeration, the reactive surface area is only partially available and relatively high starting quantities of zinc oxide therefore have to be used to obtain the required effect of HCl absorption.
The prior art describes the use of nano zinc oxide dispersions for various applications. The term “nanoparticles” generally refers to particles having a diameter of less than about 100 nm.
Due to its large reactive particle surface area nanoparticulate zinc oxide is suitable for use as a catalyst. It is a particularly important activator for crosslinking polymers, and in particular rubbers, as described for example in WO02/083797. Compared with conventional zinc oxides with a smaller active surface area, comparable polymer crosslinking is obtained with considerably lower quantities of nanoparticulate zinc oxide.
Zinc oxide nanoparticles with particle sizes of less than about 30 nm are also suitable for use as UV absorbers in transparent organic/inorganic hybrid materials, plastics, lacquers and non-adhesive coatings. In addition, they can also be used for protecting UV-sensitive organic dyes and pigments.
Particles or agglomerates which are larger than about 30 nm lead to scattered light effects and thus to an undesirable reduction in transparency in the visible light range.
The problem on which the present invention is based was that of providing an aqueous, phasing-resistant adhesive composition which, after being applied to the substrates to be bonded and after joining has high resistance to the cleavage of HCl from the polymer and which is suitable for use in spraying processes such as for example the “spray-mixing” process without any blockages being formed in the nozzles.